The present invention relates in general to semiconductor laser structures which incorporate strain-compensated multiple quantum wells as the laser gain medium.
High speed lasers are useful in optical communications, optical interconnects and many military applications. As data rates for commercial communication systems increase rapidly to multi-gigabits per second, the need for ultrafast lasers (&gt;30 GHz bandwidth) will likely increase rapidly also. Vertical cavity surface emitting lasers (VCSELs) have even broader applications although the cost and performance of such devices until now has been a limiting factor. Long wavelength (e.g., 1.3 or 1.55 .mu.m) vertical cavity lasers have a single longitudinal mode characteristic similar to that of distributed feedback (DFB) lasers, these being the most important light sources for advanced fiber communication systems. Commercial DFB lasers cost $2,000 to $6,000 each because of the extremely sophisticated fabrication process and very time consuming screening test employed in their production period. Vertical cavity surface emitting lasers are easier to fabricate than DFB lasers, but have not been able to match their performance characteristics. If their performance characteristics could be substantially improved, however, the demand for such lasers will likely increase exponentially and eventually dominate the market of lasers used for fiber communications.
Recent advances in GaAs/AlGaAs VCSELs have provided the structures with a number of advantages, including low threshold current, single mode behavior, circular beam pattern, high packing density and relatively simple fabrication processing. These advantages make them particularly attractive in many applications including optical communications, optical interconnects, optical computing and displays. However, because of the material property constraints, the development of InP-based long wavelength (e.g. 1.55 .mu.m) VCSELs is far behind that of its short wavelength (0.8 to 1 .mu.m) counterpart. These constraints result from a number of difficulties including low mirror reflectivity, high cavity loss mainly due to intervalence band absorption, strong Auger recombination and large valence band discontinuity which increases the series resistance tremendously. These difficulties must be overcome if long wavelength VCSELs are to become a feasible alternative to DFB lasers in optical communications and other applications.
Meanwhile, manufacturers continue to look for ways to reduce the cost of semiconductor lasers by reducing their size and complexity. Unfortunately, the goals of increased operating speed, VCSELs which operate efficiently at longer wavelengths and reduced size and fabrication costs tend to work against each other when applied to known laser structures. As a result, there exists a substantial need for a new type of laser structure which can achieve all of these goals simultaneously.